Best tip I have is to put the diaphragm in your mouth and leave it there while you work, drive, mow lawn - whatever. Move it around with your tongue to get comfortable with it. Make small sounds and vary the note and tone. Use it to play little songs to give you a feel for all the variations in sound and how to control them. When you really start to get control of what comes out, then start trying to make bugle sounds.
Practice, practice, practice! If you're learning to play a musical instrument it takes a lot of practice to sound reasonably good. A mouth diaphragm is no different. That being said, elk can be gullible, and you don't need to be perfect.